Durene in purified form is in substantial demand as an intermediate for a variety of uses, for example, synthetic polymers, e.g., coatings for semiconductors, fibers, plastomizers, organic synthesis, etc. Readily available sources of durene are coal tar distillates or fractions resulting from catalytic processing of petroleum and alkylation of o-xylene. Normally, purification is accomplished by re-crystallization of a fraction having a narrow boiling point range and/or melting point, but a large number of theoretical stages is required. According to Chartov et al, durene could not be separated from its isomers by adsorption on zeolites (Chem. Abstract 92 (7):58328d (1972)), since the difference in critical diameters between the isomers is too small.
Campbell et al, U.S. Pat. No. 3,864,416, disclosed that 2,4,5-trimethylcumene can be separated from mixtures of tetra-alkyl-substituted benzenes with X or Y zeolites exchanged with Group I-A metals, particularly sodium, potassium and cesium. However, the 2,4,5-trimethylcumene is rejected while the other isomers are selectively adsorbed. In the present invention, 1,2,4,5-tetramethylbenzene (durene) is unexpectedly selectively adsorbed by lithium-exchanged X zeolite, whereas zeolites exchanged with ions other than lithium reject durene as might be expected from the teachings of Campbell et al.
U.S. Pat. No. 4,743,708 discloses a process for separating a C.sub.10 aromatic isomer, particularly paradiethylbenzene from a feed stream of C.sub.10 aromatic isomers by contacting the stream with the adsorbent, zeolite beta. It is also stated that durene is preferentially adsorbed over prehnitene and isodurene. However, the patentees do not teach applicants' separation with a lithium-exchanged zeolite. Patentee preferred sodium as the ion exchange cation. However, sodium-exchanged X zeolite, was unsatisfactory in applicants' separation, for the reason that durene and isodurene were coextracted and, hence, no separation was obtained. It is more advantageous to be able to separate durene by extraction on an X zeolite than on a zeolite beta because X zeolite is commercially available.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,642,397 is incorporated by reference herein.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
We have found a specific adsorbent lithium-exchanged X zeolite, which, in combination with certain aromatic desorbent liquids, will selectively adsorb durene from its isomers.